1. Field of the Invention
This invention is directed to a truncoconical spring. More particularly, this invention is directed to a double truncoconical spring of wire with a circular cross section in which the coil diameter of the spring is greatest in the middle portion of the spring and in which at maximum load the coils of one half of the spring lie within one another in the form of a spiral and which has a linear or partially linear and partially progressive characteristic.
2. Discussion of Prior Art
Truncoconical springs, in which the coils at maximum load line inside one another in the form of a spiral, are in the prior art. In comparison with cylindrical coil springs they have the advantage of lower structural height, especially in the fully collapsed spring in which the structural height is the same as the wire diameter.
Double truncoconical springs are also in the prior art. In comparison with springs having the shape of a single frustum of a cone they basically have the advantage that a better utilization of space and material can be achieved with them. This is to be attributed to the fact that, in the double truncoconical spring the two dead end coils have a small diameter and thus require a small volume of material. Due to the double utilization of the space between the maximum spring diameter and the minimum spring diameter, the outside diameter of the spring also becomes smaller than the outside diameter of a spring of the same characteristic having the form of a single frustum of a cone.
In spite of these advantages, double truncoconical springs hitherto have found very few applications because they have a number of serious disadvantages which have not been overcome until now.
Initially, the requirement that the coils lie inside one another and the requirement that the spring have a certain characteristic cannot be simultaneously satisfied. The double truncoconical spring, therefore, if the smallest possible coil diameter and minimal weight are also to be achieved, has a typical progressive characteristic which is unsuited to most of the practical applications involved.
Another important disadvantage of the known springs of this kind lies in poor utilization of material. The efficiency of material utilization in a helical spring is expressed, under otherwise equal conditions, by the formula: ##EQU1## V represents the volume of the material and .tau. the torsional stressing of the particular coil. Since the volume is proportional to the diameter of the coil and proportional to the square of the wire diameter, it follows that the efficiency of material utilization of a truncoconical spring or of a double truncoconical spring diminishes very greatly as the coil diameter decreases, doing so as the square of the ratio of the largest coil diameter to the smallest. In practice, this means that a spring in which the ratio of the largest coil diameter to the smallest coil diameter is 5:1, and in which the efficiency of material use is virtually optimum at the largest coil diameter, in the smallest coil diameter the efficiency of material use will amount to only 1/25th. Therefore, in all coils which are smaller than the largest, the material utilization is imperfect, and if optimum material use were achieved the spring would have a substantially smaller volume of material and thus also a substantially lighter weight, and, as a further consequence of a the greater efficiency of material use, it would have a small number of turns and a smaller outside diameter.
Another serious disadvantage of the prior-art double truncoconical springs having a progressive characteristic consists in the fact that, when the spring is fully collapsed, the coils with the greatest diameter located in the middle of the spring are the first to be neutralized. This makes the spring entirely unsuitable for most constructional purposes, because the coils that are the first to be neutralized have no clearly defined bearing. Consequently the tensions in the material may be exceeded and the life of the springs may be shortened. In addition, spring coils freely oscillating in the center of the spring cause considerable noise, which has hitherto made the use of such springs impossible, for example, in motor vehicle construction.